390 ME. W. HOPKIXS OX THE COXSTEUCTIOX OF A XETF CALOEDIETEE 
Hence we obtain 
or since, in the present case, h is very small, we have approximately 
the real temperature bf the radiating surface. Hence if Qi denote the obsen'ed quan- 
tity of heat which radiates from the disc, and Q the quantity which would radiate if the 
temperature of the disc’s upper surface were #i, we have 
and therefore 
Q=Q.{i+(i-9t} 
very nearly. The second term, Qj fl — is the correction required. 
In my former experiments on conductivity, - was the quantity determined, c being the 
radiating power of mercury in air, the upper surface of the substance experimented on 
having been always covered with mercury, from which, consequently, the radiation took 
place. The above correction may be put in the form (somewhat more convenient for 
application) of 
^■ 0-0 
he cj- 
k c 
It is always a very small quantity. 
10. Another correction requires to be carefully examined. In order that the calori- 
meter should receive directly on its blackened lower surface the whole of the heat 
emanating from the heated radiating surface, it would be necessary that the cu'cum- 
ference or rim of its lower surface should coincide with the circumference of the 
radiating disc. But this condition cannot be accurately satisfied without the risk, as 
already explained, of deranging the experiments by heat passing to the calorimeter by 
conduction. It was to avoid this the more effectively that the wooden annulus (fig. C) 
was introduced, and thus a small interval of about the thickness of the wooden annulus 
existed between the circumference of the hollow bottom of the calorimeter and that of 
the radiating disc. This interval was equal to 0-21 inch. The heat which radiated 
on this intervening annulus would not immediately affect the calorimeter, and ought 
therefore to be added to the quantity which radiated directly on the calorimeter, and 
was measured by the indications of its thermometer. 
The quantity of heat which radiates directly from the heated disc and foil on any 
